To date, the highest conversion efficiencies achieved for monolithically-grown, series-connected, multi junction solar cells uses semiconductors with selected energy bandgaps, grown with atomic lattice spacing closely matched to that of the growth substrate. The top two subcells use GaxIn(1-x)P where x is around 0.5, giving bandgap 1.85-1.95 eV for the top subcell. The next mid-cell uses InxGa(1-x)As with low In content, providing bandgap 1.38-1.43 eV. Higher efficiencies of the germanium (Ge) subcell is replaced by a subcell with bandgap between 0.95-1.05 eV of higher content indium of InxGa(1-x)As. The solar spectrum is more effectively matched if a fourth subcell with lower bandgap is included. These two lower subcells use InxGa(1-x)As where x extends from 0.0 to 0.3. The lattice-mismatch of these lower cells to Ge or GaAs growth substrate can be between 2% and 5%, and can be potentially mitigated by lattice-grading or annealing schedules. The loss in output from lattice-mismatch is offset by the better use of the solar spectrum and the increased voltage. If the lower subcells are grown first, lattice-mismatch reduces the output of the top two cells, which provides 75-85% of the multi junction cell output. To avoid these mismatch effects on the top two subcells, inverted multi junction cells are grown, with the top subcells are grown first and lattice closely matched to the Ge or GaAs growth substrate. The lower subcells are grown with mismatched, but the improved energy gap selection can offset the loss in performance resulting from the mismatch, and some mitigation is possible.
A typical solar cell includes two semiconductor layers in facing contact at a semiconductor junction. When illuminated by the sun or otherwise, the solar cell produces a voltage across the semiconductor layers. More advanced solar cells may include three or more semiconductor layers that define multiple junctions.
The voltage and current output of a solar cell are limited by the materials of construction and the active surface area of the solar cell structure. Therefore, multiple solar cells are typically electrically interconnected, such as in series, to form a circuit that produces higher voltages than are possible with a single solar cell. A typical solar panel is formed by electrically connecting several circuits, such as in parallel or in series, to produce higher currents or higher voltages. A solar array may be formed as a combination of solar panels. Solar arrays are now used in space and terrestrial applications.
A circuit of solar cells works well when all of the solar cells are illuminated with generally the same illumination intensity. However, if one of the solar cells is shaded while the others remain fully illuminated, the shaded solar cell is subjected to a reverse-bias condition by the continuing voltage and current output of the remaining solar cells.
By-pass diodes are used to protect against the damage arising during the reverse-bias condition. A by-pass diode blocks current when the solar cell is not reverse biased, but passes the impressed current when the solar cell is reverse biased.
While certain techniques for incorporating by-pass diodes into solar cells are known, those skilled in the art continue to seek new ways of incorporating by-pass diodes into solar cell structures.